A procedure has been developed for preparation of human placental vesicles. These vesicles are derived from the maternal surface of the syncytiotrophoblast. The vesicles have been characterized both biochemically and morphologically. This vesicle preparation is a model for the study of the effects of environmental toxicants on human tissue. The objective of the study is to determine the relationship between heavy metal-induced placental toxicity and the mechanism of heavy metal-induced fetotoxicity. Mercury and cadmium directly affect the human placental vesicle membrane. These heavy metals alter the facilitated diffusion of neutral amino acids. A 30% reduction in amino acid transport is seen following exposure to the heavy metals. Mercury and cadmium are two environmental contaminants possessing potential for deleterious effects on the human fetus. The mechanism of action of mercury and cadmium--whether directly on the differentiating embryonic tissue, indirectly through action on the maternal and placental tissues or a combination of both--remains to be elucidated. Cadmium and mercury perturbs the transmembrane transport of nutrients, such as amino acids, across the placenta to the fetus. This heavy metal-induced placental toxicity may be a contributing mechanism to the fetotoxic and fetocidal effects of these environmental toxins.